Astrozombies Must Die! provides a quick fix adventure for someone looking to fill a single session. I ran a game of it in about 2-hours with some snipping of the content, so you could easily make it 3 - 4 hours - which makes it perfect for a convention demo. While written for the Fate driven Bulldog! system, the adventure will work with any flavour of Fate (or Fudge for that matter) - I used Fate Accelerated.
The adventure presents a premise and a loose structure of how the player characters can get from the start, out to the stranded ship, then slowly uncover what happened and the threat standing between them and returning the cargo to it's rightful owner. The game includes stats for the Astrozombies, their Captain and Engineer - in four different flavours of increasingly power and difficulty to handle. I ran with the Fresh Meat (the lowest), but would have opted for Trouble or greater if I had more than 2 hours to spare. It means you can adapt to your group size, expertise with Fate, and desire for challenge.
One added bonus - I found the cover illustration really handy as a prop. I printed it out, used tacky glue to apply to cardboard (from a cereal box) and then cut it out as a standee. When the Astrozombies reveal themselves to the characters for the first time - voila! - you have a visual reference so that they know what they're up against (though the descriptions in the text reverse the way the image works - with a coloured skull inside a roiling black flame).
Great and FREE! What more can you ask for?